SE540334C2

SE540334C2 - Method and system for selecting gear in a vehicle

Info

Publication number: SE540334C2
Application number: SE1551339A
Authority: SE
Inventors: Kourie Afram
Original assignee: Scania Cv Ab
Priority date: 2015-10-16
Filing date: 2015-10-16
Publication date: 2018-07-03
Also published as: DE102016011621A1; SE1551339A1

Description

METHOD AND SYSTEM FOR SELECTING GEAR IN A VEHICLE Field of the invention The present invention relates to vehicles, and in particular to a method and system for selecting gear in a vehicle. The present invention also relates to a vehicle, as well as a computer program and a computer program product that implement the method according to the invention.

Background of the invention There exist various kinds of vehicle powertrains. For example, vehicle transmissions can be of an automatic kind, where a vehicle control system completely controls gear changing operations. The gearboxes being used in these systems can, for example, include conventional automatic transmissions comprising torque converters, but may also include automated manual transmissions where the vehicle control system automatically controls gear shifting in “manual” gearboxes.

Automatic gear shifting in commercial (heavy) vehicles, in particular long-haul vehicles, often includes a control system controlled gearshift of manual gearboxes, i.e. automatic control of gearboxes comprising a discrete number of fixed gear ratios (i.e. gears). During operation of vehicles of this kind, the gearbox is often controlled by the vehicle control system in a manner in which the gear ratio is selected in dependence of the current driving conditions.

In general, it is often desirable to propel the vehicle using as high a gear as possible (i.e. the lowest possible gear ratio given the circumstances), in order to reduce the rotational speed of the power source, oftentimes a combustion engine, thereby reducing fuel consumption. Consequently, when selecting gear ratio, fuel consumption is often a matter of concern, but at the same time the vehicle should, in general, be driven using a gear ratio that is capable of propelling the vehicle in a desired manner.

For example, when going uphill, in particular when the vehicle is heavily loaded, it is often required that the gearbox of the vehicle is set to a gear ratio that is capable of providing enough power to propel the vehicle without undesired retardation.

Summary of the invention It is an object of the present invention to provide a method and system that improves the control of a selection of gear ratio when to change from a lower gear ratio to a higher gear ratio in situations when the vehicle is decelerating, i.e. is subjected to an acceleration in a direction opposite a direction of travel of the vehicle. This object is achieved by a method according to claim 1.

According to the present invention, it is provided a method for selecting gear ratio in a gearbox of a vehicle, said gearbox being arranged for transmitting power from a power source to at least one drive wheel of said vehicle, said gearbox comprising a plurality of selectable gear ratios, a change of gear ratio being controlled by a vehicle control system, said change of gear being dependent on a rotational speed of said power source. The method includes, when to change from a first gear ratio to a second gear ratio, said second gear ratio being higher than said first gear ratio: - determining a deceleration of said vehicle, and - controlling the dependency of the rotational speed of said power source when selecting gear ratio at least partly on the basis of said determined deceleration.

With regard to vehicles in general, gearboxes of the kind that historically has been used in vehicles having a manual transmission, but where a change of gear is performed automatically by means of the vehicle control system, are presently often used. This applies, e.g., both to heavy vehicles and passenger cars. Such gearboxes are usually arranged to comprise a discrete number of distinct gear ratios (gears), where the gear ratios of gearboxes in heavy vehicles often are distributed such that one or more of the lowest gear ratios (highest gears) are capable of propelling the vehicle at a constant speed when the vehicle is travelling on substantially level ground.

When vehicles of this kind, at least when being heavily loaded, are travelling uphill, a change of gear to a higher gear ratio (lower gear) is often required in order to provide enough power to properly overtake the uphill section of road in a desired manner. In particular, when travelling uphill, there is often a minimum gear ratio below which the vehicle will lose speed due to lack of power for propelling the vehicle.

If a change of gear is not performed at least to the highest gear (the lowest gear ratio) where this criterion is fulfilled, the vehicle will continue to decelerate due to the propelling force being less than the driving resistance. The highest gear where enough power is obtained can, for example, be denoted as “Gear With Power to Accelerate”, in the following referred to as GWPA gear.

For this reason, a change of gear ratio to a gear ratio being equal to or above the GWPA gear ratio is common in order to be capable of overtaking the uphill section of road without losing vehicle speed other than what is required to fulfil criteria for engaging the GWPA gear. For this reason, when a heavy vehicle enters an uphill section of road, the vehicle is often allowed to decelerate as a change of gear towards the GWPA gear is performed, e.g. through intermediate changes of gear to intermediate gear ratios.

A change of gear is often performed when one or more criteria’s are fulfilled. For example, a change of gear to a lower gear (higher gear ratio) can be arranged to be performed when it is determined that the rotational speed of the combustion engine following the change of gear will not exceed some predetermined calibrated speed limit that is not to be exceeded, possibly by the further criterion that the vehicle is expected to be propelled on the new gear for at least a minimum period of time before the rotational speed of the combustion engine has reached a rotational speed that again requires a change of gear to a lower gear. The calibrated power source speed can, for example, be different for different gears and/or vehicle speeds.

This strategy for changing gear, however, may result in changes of gear to lower gears in situations where the change of gear in reality may not be motivated. For example, the deceleration of the vehicle on a current gear may be relatively low, i.e. the vehicle may only slowly be losing speed. If a change of gear is performed in a situation of this kind, this may seem unnecessary and discomforting to the driver, and the change of gear may also be disadvantageous from a fuel consumption point of view.

According to the present invention, situations of this kind can be avoided or at least be reduced in occurrence. This is accomplished by means of a method where a deceleration of the vehicle is determined, and where the dependency of the rotational speed of the power source when changing gear is controlled at least partly on the basis of said deceleration. For example, the maximum allowed rotational speed of said power source following the change of gear can be controlled in dependence of the determined deceleration.

Consequently, selection of gear is made dependent of the deceleration of the vehicle. When the vehicle is travelling in an uphill section of road, different road inclinations will result in different decelerations, where lower inclinations will result in less deceleration than higher inclinations for a similarly loaded vehicle, and vice versa. According to the present method, the vehicle can be arranged, for example, to continue on a current gear for a longer period of time than previously, thereby allowing propulsion on lower power source rotational speeds when the vehicle deceleration is below e.g. a predetermined deceleration.

This means that selection of gear, and thereby change of gear, for a same vehicle speed can be different for different situations in which the vehicle is decelerating due to insufficient power to overcome the current driving resistance. For example, the vehicle can be arranged to be propelled longer on a current gear for lower uphill inclinations than for higher inclinations. In this way, for example, changes of gear that normally occur also when the vehicle is only slowly losing speed may be avoided or at least postponed.

It is also possible to increase the maximum allowed rotational speed on the basis of the deceleration. For example, the maximum allowed rotational speed of the power source can be increased to a value exceeding the calibrated maximum allowed rotational speed when the deceleration exceeds some predetermined value, which can be similar to the predetermined deceleration mentioned above or a higher deceleration, and where the increase can be dependent on the magnitude of the deceleration.

Consequently, the vehicle can be arranged to automatically adapt to the current driving conditions in a manner that may be perceived as logical to a driver of the vehicle. If the deceleration is higher, selection of gear can be performed such that change of gear occurs earlier, while if the deceleration is lower operation on a current gear can be prolonged. Vehicle load may also be taken into account, since different vehicle loads will result in different decelerations.

For example, the maximum allowed rotational speed of the power source following the change of gear can be arranged to be reduced more for lower decelerations than for comparatively higher decelerations. Consequently, the lower the deceleration, the longer the vehicle may be allowed to continue on a current gear before a change of gear is performed.

According to one embodiment, the vehicle is, in addition or alternatively, forced to maintain propulsion on a current gear for as long as the rotational speed of the power source has not fallen below a minimum speed limit. For example, change of gear ratio from said first gear ratio to said second gear ratio may be prohibited for as long as the rotational speed of said power source when said first gear ratio is engaged is above said rotational speed limit.

This minimum speed limit may also be arranged to be dependent on the determined deceleration, where lower limits of the rotational speed can be applied for comparatively lower decelerations. Basically this has the same effect as the embodiment exemplified above, since the difference in rotational speed of the power source before and after a change of gear is linked through the difference in gear ratio between the gears.

Further characteristics of embodiments of the present invention and advantages thereof are indicated in the detailed description of exemplary embodiments set out below and the attached drawings.

Brief description of the drawings Fig. 1 A illustrates a powertrain of an exemplary vehicle; Fig. 1 B illustrates an example of a control unit in a vehicle control system; Fig. 2 illustrates an exemplary method according to an embodiment of the present invention; Fig. 3 illustrates a gear changing scenario according to the prior art; Fig. 4 illustrates a gear changing scenario according to an embodiment of the present invention.

Detailed description of exemplary embodiments Fig. 1 A schematically depicts a powertrain of an exemplary vehicle 100. The powertrain comprises a power source, in the present example a combustion engine 101 , which, in a conventional manner, is connected via an output shaft of the combustion engine 101 , normally via a flywheel 102, to a gearbox 103 via a clutch 106. An output shaft 107 from the gearbox 103 propels drive wheels 113, 114 via a final drive 108, such as a common differential, and drive axles 104, 105 connected to said final drive 108.

The combustion engine 101 is controlled by the vehicle control system via a control unit 115. The clutch 106 and gearbox 103 are also controlled by the vehicle control system by means of a control unit 116.

The gearbox 103 can be set to a plurality of distinct (discrete) gear ratios, where a suitable gear ratio is selected for each driving situation by means of the vehicle control system. As was mentioned above, there exist situations where the control system selection of a particular gear ratio may not be perceived as being motivated by the vehicle driver. Embodiments of the invention provide a method for changing gear ratio in a decelerating vehicle that may seem more appropriate from a driver perspective.

An exemplary method 200 according to embodiments of the present invention is shown in fig. 2, which method can be implemented at least partly e.g. in the control unit 116 for controlling the clutch 106 and gearbox 103. As indicated above, the functions of a vehicle are, in general, controlled by a number of control units, and control systems in vehicles of the disclosed kind generally comprise a communication bus system including one or more communication buses for connecting a number of electronic control units (ECUs), or controllers, to various components on board the vehicle. Such a control system may comprise a large number of control units, and the control of a specific function may be divided between two or more of them.

For the sake of simplicity, Fig. 1 A depicts only control units 115-116, but vehicles 100 of the illustrated kind are often provided with significantly more control units, as one skilled in the art will appreciate. Control units 115-116 are arranged to communicate with one another and various components via said communication bus system and other wiring, partly indicated by interconnecting lines in fig. 1 A.

Embodiments of the present invention can be implemented in any suitable control unit in the vehicle 100 and hence not necessarily in the control unit 115. The control of the gearbox 103 according to embodiments of the present invention will usually depend on signals being received from other control units and/or vehicle components, and it is generally the case that control units of the disclosed type are normally adapted to receive sensor signals from various parts of the vehicle 100. The control unit 116 will, for example, receive signals from the engine control unit 115. Control units of the illustrated type are also usually adapted to deliver control signals to various parts and components of the vehicle, e.g. to control the clutch 106 and gearbox 103.

Control of this kind is often accomplished by programmed instructions. The programmed instructions typically include a computer program which, when executed in a computer or control unit, causes the computer/control unit to exercise the desired control, such as method steps according to embodiments of the present invention. The computer program usually constitutes a part of a computer program product, where said computer program product comprises a suitable storage medium 121 (see Fig. IB) with the computer program 126 stored on said storage medium 121. The computer program can be stored in a non-volatile manner on said storage medium. The digital storage medium 121 can, for example, include any of the group comprising: ROM (Read-Only Memory), PROM (Programmable Read-Only Memory), EPROM (Erasable PROM), Flash memory, EEPROM (Electrically Erasable PROM), a hard disk unit etc, and be arranged in or in connection with the control unit, whereupon the computer program is executed by the control unit. The behaviour of the vehicle in a specific situation can thus be adapted by modifying the instructions of the computer program.

An exemplary control unit (the control unit 116) is shown schematically in Fig. 1 B, wherein the control unit can comprise a processing unit 120, which can include, for example, any suitable type of processor or microcomputer, such as a circuit for digital signal processing (Digital Signal Processor, DSP) or a circuit with a predetermined specific function (Application Specific Integrated Circuit, ASIC). The processing unit 120 is connected to a memory unit 121 , which provides the processing unit 120, with e.g. the stored program code 126 and/or the stored data that the processing unit 120 requires to be able to perform calculations. The processing unit 120 is also arranged so as to store partial or final results of calculations in the memory unit 121.

Furthermore, the control unit 112 is equipped with devices 122, 123, 124, 125 for receiving and transmitting input and output signals, respectively. These input and output signals can comprise waveforms, pulses or other attributes that the devices 122, 125 for receiving input signals can detect as information for processing by the processing unit 120. The devices 123, 124 for transmitting output signals are arranged so as to convert calculation results from the processing unit 120 into output signals for transfer to other parts of the vehicle control system and/or the component (s) for which the signals are intended. Each and every one of the connections to the devices for receiving and transmitting respective input and output signals can include one or more of a cable; a data bus, such as a CAN bus (Controller Area Network bus), a MOST bus (Media Oriented Systems Transport) or any other bus configuration, or of a wireless connection.

Returning to the exemplary method 200 illustrated in fig. 2, the method starts in step 201 , where it is determined whether there is a control system initiated request for a change of gear. This can, for example, be determined by determining whether some suitable function in the vehicle control system determines that the current state of driving has reached a situation where a gear changing operation should be performed. This can be determined e.g. according to any suitable function for determining when a gear change is to be performed. Embodiments of the invention relate to changes of gear to higher gear ratios (lower gears), and hence, according to one embodiment, it can be determined in step 201 whether the request for change of gear is a request for a change of gear to a lower gear (higher gear ratio). According to one embodiment, it is also determined whether there is an ongoing deceleration that is due to driving resistance of the vehicle.

When it is determined that a change of gear is to be performed, the method continues to step 202, where a current deceleration dec of the vehicle 100 is determined. The vehicle deceleration dec can be determined by means of any suitable means as is known per se, such as e.g. by the use of an accelerometer, or by determining a reduction in speed between two consecutive points in time. In step 203 it is determined whether the vehicle deceleration dec is below a deceleration limit declim. If this is not the case, i.e. when the vehicle deceleration dec is above the deceleration limit declimthe method continues to step 204 where e.g. a normal gear changing operation is initiated. This change of gear can be arranged to be carried out according to the criteria that normally are applied in the propulsion of the vehicle 100, e.g. with regard to restrictions regarding combustion engine speed before/after a change of gear etc.

However, if it is determined in step 203 that the vehicle deceleration dec is below said deceleration limit declim, the method continues to step 205. In step 205 gear changing criteria regarding combustion engine speed n are adapted such that the change of gear is delayed, i.e. the vehicle 100 is forced to be propelled on the current gear for a longer period of time than normally is the case. This can, for example, be accomplished by reducing the highest rotational speed nlimof the combustion engine 101 that is allowed following the change of gear. In this way the criteria for performing the gear changing operation will not be fulfilled until the vehicle 100 has decelerated further and the vehicle 100 thereby being propelled for a longer period of time on the current gear. The effect of embodiments of the invention is explained further with reference to figs. 3-4.

Fig. 3 exemplifies a normal scenario during a gear changing operation. Fig. 3 shows the combustion engine 101 rotational speed n as a function of time t. The figure further discloses the change in combustion engine speed n for two different scenarios, the dash-dotted line 301 representing the change in combustion engine speed n in a situation where the vehicle 100 loses vehicle speed, and hence combustion engine rotational speed n , relatively fast. This can be the case, for example, when the vehicle 100 is being driven in a steep uphill section of a road, in particular when being heavily loaded.

The solid line 302 represents the change in combustion engine speed n for a situation where the vehicle 100 is being driven in a relatively less steep uphill section of road and/or being loaded to a lesser extent. The dashed line 303 represents the combustion engine speed limit nlim that is not to be exceeded following a change of gear to a lower gear (i.e. change of gear ratio to a higher gear ratio).

Starting from time to, in the situation being represented by dash-dotted line 301 , the vehicle 100 is continuously losing speed (decelerating) and hence the rotational speed n of the combustion engine 101 is also continuously reducing.

At time ti the combustion engine 101 speed n reaches a speed nchange, representing a time/speed where conditions for a change of gear to a lower gear (higher gear ratio) are fulfilled. As was mentioned above, a condition for performing a change of gear is often that the speed of the combustion engine following a completed change of gear is not allowed to exceed some speed limit nlim, which often is a calibrated maximum engine speed that is not to be exceeded. This maximum calibrated engine speed nlim can, for example, be different for different gears and/or vehicle speeds. As a consequence, a change of gear to a lower gear is, in general, performed when it is determined that the speed of the combustion engine on the new gear will not exceed said speed limit nlim. This has the result that, following the change of gear, the speed of the vehicle combustion engine is increased to a speed limit equal to or less than nlimat time t2.

Oftentimes, when changing gears, a further condition regarding the change of gear is also utilized. This, too, is illustrated in fig. 3, where a further determination is made prior to the actual change of gear. Specifically it is determined whether the vehicle 100 is expected to be driven on the new (lower) gear at least for a time txbefore a further change of gear is initiated. That is, it is determined whether the vehicle is expected to be driven on the new gear at least for the time txuntil the speed n of the combustion engine 101 again reaches e.g. the speed nchangeat which a change of gear to a lower gear is again initiated. When this is the case, a change of gear is performed to the intended gear, while otherwise a change of gear to an even lower gear may be evaluated instead.

According to the prior art, this change of gear is performed irrespective of the rather substantial differences in driving conditions that in reality may prevail when a change of gear is to be carried out. When the speed of the vehicle is dropping relatively fast, as is e.g. the case with the situation represented by dashed line 301 , a change of gear according to fig. 3 is often highly desirable in order to reduce the deceleration. The change of gear makes available higher drive wheel power, so that the vehicle will be losing speed more slowly, or even be capable to accelerate.

However, as is illustrated by solid line 302, the situation can also be different. In the situation being represented by solid line 302 the vehicle 100 is decelerating more slowly, i.e. loses speed more slowly than in the situation represented by dashed line 301. Still the vehicle 100 is subjected to the same combustion engine 101 speed criteria regarding change of gear. Hence it will be determined at time ti that a change of gear is to be performed in this situation as well. A change of gear in a situation of this kind, however, may seem highly uncalled for, at least from a driver perspective.

According to embodiments of the invention, it is provided a method that distinguishes between situations of this kind through the use of the determination of a deceleration according to the above. This is illustrated in fig. 4, which shows a similar situation as in fig. 3, where the dash-dotted line 401 is similar to the dashed line 301 in fig. 3. With regard to the situation represented by dash-dotted line 401 it is determined in step 203 that the deceleration dec of the vehicle 100 determined in step 202 is above the limit declim. Therefore, similar to the above, a change of gear is still performed at time t2 in this case.

With regard to the scenario represented by solid line 402 in fig. 4, on the other hand, the situation is different. In this case, it is determined in step 203 that the deceleration dec of the vehicle is below said limit declim. Therefore, even though the requirements for a change of gear in principle are fulfilled at time t1, the change of gear is not performed but delayed. This is accomplished by lowering the maximum allowed combustion engine speed nlim following a change of gear to a second speed limit nlim_dec, step 205. The speed limit nlim_deccan be determined, for example, in dependence of the current deceleration of the vehicle 100. That is, the difference ?n between the speed limit nlimand the new limit nlim_deccan be arranged to be different for different decelerations. Consequently, in the situation according to the solid line 402, when the vehicle 100 reaches time t2, no change of gear is performed since the new gear changing criteria are not fulfilled. Instead, the vehicle 100 is allowed to continue on the current gear while the vehicle speed, and thereby combustion engine speed n , continues to reduce. At time t3it is determined that the speed of the combustion engine 101 following a change of gear to a lower gear will be below the second speed limit nlim_dec. A change of gear is then performed at time t4. In step 206 it is determined whether the new criteria for changing gear are fulfilled, and the method can be arranged to remain instep 206 for as long as this is not the case or until the method of fig. 2 is aborted, e.g. due to a change of gear no longer being required, and controlled by some other vehicle function. Otherwise a change of gear is performed in step 207 and the method can then return to step 201 waiting for a new request for change of gear.

Consequently, according to embodiments of the invention, the vehicle 100 is allowed to continue on a current gear for a longer period of time than otherwise is allowed. This means that the vehicle 100 in at least some situations may behave more according to the expectations of the driver and not perform changes of gear e.g. in situations where this may not be necessary.

For example, the vehicle 100 may be only very slowly losing speed. According to the prior art a change of gear will still take place when the set criteria are fulfilled. Such vehicle behaviour can be difficult to comprehend, in particular when the vehicle deceleration is so low that it may appear as if the vehicle 100 is travelling at essentially constant speed. The road may also e.g. be about to level out. A change of gear to a lower gear in a situation of this kind may even seem contradictive to an expected change of gear, since a change of gear to a higher gear may be more appropriate when the road levels out.

Furthermore, instead of, or in addition to, reducing the maximum allowed combustion engine speed following the change of gear, a further/other criterion can be set. For example, the criterion can instead be set such that the vehicle is forced to continue on a current gear until the speed n of the combustion engine 101 has reduced to a speed nchange_dec, see fig. 4, that is lower than the combustion engine speed nchangeat which change of gear normally is carried out.

According to embodiments of the invention, consequently, the occurrence of undesired changes of gear can be reduced. Embodiments of the invention further has the advantage that the vehicle may be perceived as being more powerful since the vehicle, on average, will be driven at lower combustion engine speeds. The reduction in average combustion engine speed also reduces fuel consumption.

There may exist, however, situations where it can be desired that this aspect of the invention is not carried out. According to one embodiment of the present invention, therefore, a further criterion is applied in order to allow the reduction of the maximum allowed engine speed of the combustion engine. That is, the maximum allowed combustion engine speed is only reduced if this further criterion is fulfilled.

This further criterion can, for example, include a requirement that the total reduction in speed exhibited by the vehicle 100 since the reduction in speed began must not exceed some suitable speed difference. When a heavily loaded vehicle enters e.g. a steep hill and/or an uphill section of road of some length a change of gear with associated required reduction in speed is often inevitable. This reduction in speed can be substantial for a heavily loaded vehicle, at least in longer uphill sections of road. It is, therefore, sometimes desirable, from a driver point of view, that the vehicle is being propelled using the maximum deliverable power from the combustion engine in order to reduce the loss in vehicle speed as much as possible.

Embodiments of the invention have the effect that the vehicle on average is propelled at lower combustion engine speeds, and hence at a lower average combustion engine power. Embodiments of the invention may therefore cause the vehicle to suffer an additional reduction in speed that is not desired.

Consequently, there may exist situations where a delay of a change of gear to a lower gear by a continued propelling of the vehicle on a current gear according to the above is not desirable since this increases further an already substantial reduction in speed when overtaking the uphill section of road.

In order to avoid such situations from occurring, therefore, according to one embodiment of the invention, a determination is made of the total loss in speed that the vehicle has suffered since the loss in speed began. This can be determined, e.g., by some suitable function in the vehicle control system. For example, vehicle speed, at least with regard to heavy vehicles, is, in general, logged throughout the journey and hence available, both present value and past, on the vehicle communication bus system.

It is also possible to use a dedicated function that monitors the vehicle speed and generates a signal, e.g. when the vehicle speed starts reducing, and/or when the vehicle speed has reduced to a suitable extent where the delayed change of gear should no longer be utilised.

According to one embodiment, it is also determined if the reduction in speed corresponds to a corresponding increase driving resistance and not through the use of a vehicle braking system. Also this can be determined in any suitable manner as is known per se and as oftentimes already is determined for use in various other existing vehicle functions.

It can be determined whether the vehicle speed has been reduced by some suitable speed difference, such as e.g. a percentage of the vehicle speed prevailing when the reduction in speed began or some suitable number of km/h. Other criteria can also be used. If the vehicle speed has been reduced to such an extent that a further reduction in speed is not desirable, the use of the delayed change of gear according to the above can be prohibited.

Above, embodiments of the invention have been described where the deceleration is used to lower e.g. the maximum allowed rotational speed of a power source following a change of gear. According to embodiments of the invention, e.g. the maximum allowed rotational speed can also be increased on the basis of the deceleration. For example, in correspondence to the above, the calibrated maximum allowed rotational speed of the power source can be increased when the deceleration exceeds some predetermined value, which can be the same or higher than the deceleration at which the maximum allowed rotational speed is lowered. Also the increase can be dependent on the magnitude of the deceleration. Consequently, a maximum allowed rotational speed that has already been lowered can be increased when the deceleration increases, and vice versa.

Furthermore, embodiments of the invention can be used in combination with the solution described in the parallel Swedish patent application having the same filing date as the present application, the same inventor and the title “METHOD AND SYSTEM FOR CONTROLLING SELECTION OF GEAR RATIO IN A VEHICLE”.

This parallel application relates to a method and system where undesired losses in vehicle speed due to the driving resistance exceeding the available propelling power can be reduced. This is accomplished by means of a method where, when it is determined that the vehicle e.g. has lost speed to some extent, caused by the driving resistance exceeding the propelling force, the dependency of the rotational speed of the vehicle power source when selecting gear is controlled on the basis of the loss in speed of the vehicle.

For example, the rotational speed of the power source at which a change of gear is carried out, and/or the resulting rotational speed of the power source following the change of gear, can be increased irrespective of whether the deceleration is low. This allows the vehicle to be propelled at higher rotational speeds of the power source, and thereby higher power to reduce the deceleration and continued loss in vehicle speed. The increase in allowed rotational speed of the power source can be arranged to depend on the reduction in speed that the vehicle has suffered, and also on the deceleration of the vehicle. Consequently, the higher the loss in speed, the higher rotational speeds of the power source may be allowed.

Embodiments of the invention can be used in combination with the solution described in the parallel application, so that the allowed power source rotational speeds when changing gear can be adapted according to current conditions. For example, control can e.g. be performed according to the present application for as long as the loss in vehicle speed does not exceed some suitable speed difference. When the vehicle has lost too much in speed, the vehicle can, instead be controlled according to the solution described in the parallel application to reduce negative impact of loss in speed.

According to the combined solution, the vehicle can be propelled at lower combustion engine speeds, and hence at a lower average combustion engine power, when possible, but where it may be ensured that the vehicle does not lose too much speed. Further according to the combined solution, e.g. the maximum allowed rotational speed may be arranged to increase and decrease according to the current driving conditions, so that the vehicle behaviour can be adapted to prevailing conditions.

Finally, embodiments of the invention have been exemplified above for a particular example of a vehicle, but is applicable for any vehicle in which the vehicle control system controls gear change. The invention also encompasses a vehicle comprising a system implementing the invention.

Claims

1. Method for selecting gear ratio in a gearbox (103) of a vehicle (100), said gearbox (103) being arranged for transmitting power from a power source (101 ) to at least one drive wheel (113, 114) of said vehicle (100), said gearbox (103) comprising a plurality of selectable gear ratios, a change of gear ratio being controlled by a vehicle control system, said change of gear being dependent on a rotational speed (n) of said power source (101), the method being characterised in, when to change from a first gear ratio to a second gear ratio, said second gear ratio being higher than said first gear ratio: - determining a deceleration ( dec ) of said vehicle (100), and - controlling the dependency of the rotational speed (n) of said power source (101) when selecting gear ratio at least partly on the basis of said determined deceleration ( dec ), the method further including: - in dependence of said determined deceleration ( dec ), controlling a maximum allowed rotational speed ( nlim_dec) of said power source (101) following said change of gear ratio in relation to a predetermined maximum allowed rotational speed.

2. Method according to claim 1 , further including: - in dependence of said determined deceleration ( dec ), reducing the maximum allowed rotational speed ( nlim_dec) of said power source (101 ) following said change of gear ratio in relation to a predetermined maximum allowed rotational speed.

3. Method according to claim 1 or 2, further including: - controlling the maximum allowed rotational speed ( nlim_dec) of said power source (101) following said change of gear ratio in relation to a predetermined maximum allowed rotational speed such that the maximum allowed rotational speed ( nlim_dec) is reduced compared to said predetermined maximum allowed rotational speed to a first rotational speed for a first deceleration and a second rotational speed for a second deceleration, said first rotational speed being lower than said second rotational speed when said first deceleration is lower than said second deceleration.

4. Method according to any one of the preceding claims, further including: - on the basis of said determined deceleration ( dec ), determining a maximum allowed rotational speed ( nlim_dec) of said power source (101), to be caused if said second gear ratio is engaged, and - initiating said change of gear ratio only when the rotational speed of said power source (101) will be below said maximum allowed rotational speed ( nlim_dec) upon engagement of said second gear ratio.

5. Method according to any one of the preceding claims, further including: - initiating a change of gear from said first gear ratio to said second gear ratio when the rotational speed (n) of said first power source (101) when said first gear ratio is engaged falls to a minimum rotational speed limit (nchange _dec), said minimum rotational speed limit (nchange _dec) being determined in relation to a predetermined minimum rotational speed limit at least partially on the basis of said determined deceleration ( dec ).

6. Method according to claim 5, further including: - controlling said minimum rotational speed limit (nchange _dec) such that said minimum rotational speed limit (nchange _dec) is reduced compared to said predetermined minimum rotational speed limit to a third rotational speed for a first deceleration and a fourth rotational speed for a second deceleration, said third rotational speed being lower than said fourth rotational speed when said first deceleration is lower than said second deceleration.

7. Method according to any one of the preceding claims, further including: - controlling the dependency of the rotational speed of said power source (101) when selecting gear ratio at least partly on the basis of said determined deceleration ( dec ) only when said determined deceleration ( dec ) is below a first deceleration limit (declim).

8. Method according to any one of the preceding claims, further including, prior to said selection of gear ratio: - determining a first vehicle speed at which a deceleration of said vehicle begins, and - initiating said change of gear ratio to said second gear ratio only when the difference in speed of said vehicle (100) in relation to said first vehicle speed is below a first speed difference.

9. Method according to any one of the preceding claims, further including: - determining whether said determined deceleration ( dec ) of said vehicle is caused by an increase in driving resistance, and - controlling the dependency of the rotational speed of said power source (101) when selecting gear ratio at least partly on the basis of said determined deceleration ( dec ) only when said determined deceleration ( dec ) of said vehicle (100) is caused by an increase in driving resistance.

10. Method according to any one of the preceding claims, further including: - controlling the maximum allowed rotational speed ( nlim_dec) of said power source (101) to follow said change of gear ratio in relation to a predetermined maximum allowed rotational speed such that the maximum allowed rotational speed ( nlim_dec) is reduced compared to said predetermined maximum allowed rotational speed when said determined deceleration ( dec ) is below a first deceleration, and such that the maximum allowed rotational speed ( nlim_dec) is increased compared to said predetermined maximum allowed rotational speed when said determined deceleration ( dec ) is above a second deceleration, the second deceleration being higher than the first deceleration.

11. Computer program comprising program code that, when said program code is executed in a computer, enables said computer to carry out the method according to any one of the preceding claims 1-10.

12. Computer program product comprising a computer-readable medium and a computer program according to claim 11 , wherein said computer program is contained in said computer-readable medium.

13. System for selecting gear ratio in a gearbox (103) of a vehicle (100), said gearbox (103) being arranged for transmitting power from a power source (101 ) to at least one drive wheel (113, 114) of said vehicle (100), said gearbox (103) comprising a plurality of selectable gear ratios, a change of gear ratio being controlled by a vehicle control system, said change of gear being dependent on a rotational speed of said power source (101), the system being characterised in that the system includes means for, when to change from a first gear ratio to a second gear ratio, said second gear ratio being higher than said first gear ratio: - determining a deceleration ( dec ) of said vehicle (100), - controlling the dependency of the rotational speed of said power source (101) when selecting gear ratio at least partly on the basis of said determined deceleration ( dec ), and - controlling a maximum allowed rotational speed ( nlim_dec) of said power source (101) following said change of gear ratio in relation to a predetermined maximum allowed rotational speed in dependence of said determined deceleration ( dec ).

14. Vehicle, characterised in that it comprises a system according to claim 13.